1. Field of the Invention
The present invention relates to an electroluminescence display device, and more particularly, to an electroluminescence display device that may prevent brightness non-uniformity due to a voltage drop in a display region.
2. Discussion of the Background
Various flat panel display devices have been recently developed to replace the conventional cathode ray tube (CRT). The flat panel display devices may be emissive or non-emissive types. The emissive types, which do not require a light source, include flat CRTs, plasma display panel devices, vacuum fluorescent display devices, field emission display devices, and inorganic/organic electro-luminescent display devices, and the non-emissive type includes liquid crystal display devices. The organic electro-luminescent display device has drawn much attention since it is an emissive type device, and it may be highly efficient and does not consume a lot of power.
The organic electro-luminescent display device emits light having a specific wavelength by energy generated from excitons, which form when electrons and holes, injected through an anode and a cathode, recombine in an organic thin film. The organic electro-luminescent display device may operate with a low voltage, it is thin and light weight, and it may have a wide viewing angle and a quick response time.
An organic electro-luminescent unit of the organic electro-luminescent display device includes a first electrode (an anode), an intermediate layer including at least an organic emitting layer (EML), and a second electrode (a cathode) stacked on a substrate. The organic EML emits light when electrons and holes recombine to form excitons. Electrons and holes may be transferred to the organic EML to increase light emitting efficiency. Accordingly, the intermediate layer may also include an electron transport layer (ETL) disposed between the cathode and the organic EML and a hole transport layer (HTL) disposed between the anode and the organic EML. Further, a hole injection layer (HIL) can be disposed between the anode and the HTL, and an electron injection layer (EIL) can be disposed between the cathode and the ETL.
The organic electro-luminescent display device may be a passive matrix (PM) or active matrix (AM) type depending upon its driving method. In the PM display device, the anodes and the cathodes are simply arranged in columns and rows, respectively, and scanning signals may be sequentially supplied to single cathodes from a row driving circuit. Also, data signals are supplied to each pixel from a column driving circuit. On the other hand, the AM display device controls signals inputted to each pixel using a thin film transistor (TFT). Therefore, the AM device is widely used to display moving images since it can process a large number of signals.
FIG. 1A is a plan view showing a conventional AM organic electroluminescent display device, and FIG. 1B is a cross-sectional view taken along line I-I in FIG. 1A.
The AM organic electroluminescent display device of FIG. 1A has a predetermined display region 20 that includes an organic light emitting diode (OLED) on a transparent substrate 11, and a sealing member (not shown), such as a metal cap, to seal the display region 20 along with the sealing unit 80. The display region 20 comprises a plurality of pixels that include an OLED and a thin film transistor (TFT). A plurality of driving lines VDD 31 may be disposed in the display region 20. The driving lines 31 supply power to the display region 20, and they are coupled to a terminal region 70 through a driving power supply line 32, which may be disposed outside the display region 20.
As FIG. 1B shows, a TFT layer 10a, for applying electrical signals to an electroluminescent unit in the display region 20, may be formed on a surface of a substrate 11, and a pixel layer 10c, which includes an electroluminescent unit, may be formed on the TFT layer 10a. An insulating layer 10b may be interposed between the TFT layer 10a and the pixel layer 10c. The TFT layer 10a includes a semiconductor active layer 13, which may be formed on a buffer layer 12. A gate insulating layer 14 is formed on the semiconductor active layer 13 and the buffer layer 12, and a gate electrode 15 is formed on the gate insulating layer 14 at a position corresponding to a channel region of the semiconductor active layer 13. An interlayer insulator 16 may be formed on the gate electrode 15 and gate insulating layer 14, and the source and drain electrodes 17a and 17b are formed on the interlayer insulator 16.
A via hole formed in the insulating layer 10b may electrically connect the TFT layer 10a and the pixel layer 10c. FIG. 1C is a magnified cross-sectional view of portion “A” in FIG. 1B. A first insulating layer 18a may be formed on the source and drain electrodes 17a and 17b, a second insulating layer 18b may be formed on the first insulating layer 18a, and via holes 18′a and 18′b may be formed in the insulating layers 18a and 18b. 
With a front emitting electroluminescence display device, a reflection layer 19b may be formed under a first electrode layer 19a that supplies electrical signals to an intermediate layer 19c. The first electrode layer 19a and the reflection layer 19b extend to the via holes 18′a and 18′b and are electrically connected to the drain electrode 17b of the TFT layer 10a. A second electrode layer 40 may be formed over the display region.
Here, the first electrode layer 19a may be formed of a transparent conductive oxide such as, for example, indium tin oxide (ITO), having a large work function, and the reflection layer 19b may be formed of Al or AlNd. However, the transmission of electrical signals from the drain electrode 17b to the intermediate layer disposed in an opened region of a pixel defining layer 19d may be adversely affected by the reduction of conductivity by an interface oxide layer formed between the first electrode layer 19a, formed of ITO, and the metal reflection layer 19b, formed of AlNd. This can reduce brightness or cause brightness non-uniformity in the display region, thereby reducing image quality.